A steel material used for automobiles, structural materials, and the like is required to be excellent in such mechanical properties as strength, workability, and toughness. In order to improve these mechanical properties comprehensively, it is effective to make a steel material with a fine-grained structure; to this end, a number of manufacturing methods to obtain a steel material with a fine-grained structure have been sought. Further, by making the fine-grained structure, it is possible to manufacture a high strength hot-rolled steel sheet having excellent mechanical properties even if the amount of alloying elements added is reduced.
As a method for making a steel sheet with a fine-grained structure, it is known to carry out a high rolling reduction especially in the subsequent stage of hot finish rolling, deforming austenite grains greatly and increasing a dislocation density; and thereby to obtain fine-grained ferrite after cooling. Further, in view of facilitating the ferrite transformation by inhibiting recrystallization and recovery of the austenite, it is effective to cool a steel sheet to 600° C. to 700° C. as quickly as possible after rolling. In other words, subsequent to hot finish rolling, it is effective to rapidly cool a steel sheet after the rolling by arranging a cooling apparatus capable of cooling more quickly than ever before. In rapidly cooling a steel sheet after rolling in this way, it is recommended to have a large volume of cooling water sprayed over the steel sheet per unit area, and to have a high flow density in order to enhance a cooling capability.
However, if the volume of cooling water and the flow density are increased in this way, the water accumulated (i.e. retained water) on an upper surface of a steel sheet increases due to a relation between water supply and water discharge. On the other hand, on a lower surface side of a steel sheet, the retained water between a lower surface guide and the steel sheet increases. This retained water is the water which has remained after being used for cooling the steel sheet. Thus, it is desired to discharge the water as quickly as possible and to provide the steel sheet with water supplied from a cooling nozzle, thereby ensuring a cooling capability. Further, since the retained water is a layer of water, if the layer is thick, the thickness sometimes hinders water from the cooling nozzle from reaching the steel sheet effectively. Furthermore, the retained water flows from a middle portion of the steel sheet toward an end portion of the steel sheet; and the flow rate increases as the water approaches the end portion of the steel sheet. So, if the amount of retained water increases, cooling nonuniformity in a width direction of the steel sheet occurs to a large degree. In addition, if the amount of the retained water increases excessively, an end portion of the cooling nozzle sinks in the retained water on the upper surface guide.
As described above, it is effective to rapidly cool a steel sheet as soon as possible after hot finish rolling; thus cooling immediately after a work roll in a final stand of a row of hot finish rolling mills is preferable. In other words, a steel sheet is preferably cooled by spraying cooling water over the steel sheet which is inside the housing of the final stand in the row of hot finish rolling mills. This way of cooling is disclosed in Patent Document 1.